Question: A circle with center $A$ and radius three inches is tangent at $C$ to a circle with center $B$, as shown. If point $B$ is on the small circle, what is the area of the shaded region? Express your answer in terms of $\pi$.

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filldraw(circle((0,0),6),gray,linewidth(2));
filldraw(circle(3dir(-30),3),white,linewidth(2));

dot((0,0));
dot(3dir(-30));
dot(6dir(-30));

label("$B$",(0,0),NW);
label("$A$",3dir(-30),NE);
label("$C$",6dir(-30),SE);
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Explanation: Since the small circle is tangent to the large circle at $C$ and point $B$ lies on the smaller circle and is the center of the larger circle, we know the radius of the bigger circle is twice the radius of the smaller circle, or six inches.

To find the shaded area, subtract the area of the smaller circle from the area of the larger circle.  $6^2\pi - 3^2\pi = 36\pi - 9\pi = \boxed{27\pi}$. \[ - OR - \] Consider the tangent line to circle $B$ at $C$, say line $l$.  Then $BC \perp l$.  But since circle $A$ is tangent to circle $B$ at $C$, we also have that $AB \perp l$.  Hence $A$ is on segment $BC$, and $BC$ is a diameter of circle $A$.  Thus by homothety circle $A$ covers $\frac{1}{4}$ the area of circle $B$.  The shaded region is thus $\frac{3}{4}$ of the area of circle $B$, and hence is 3 times the area of circle $A$, or simply $(\pi \cdot 3^2)\cdot 3 = 27\pi$.